I would think it would be more harmful on a manual -- you have the clutch moving in opposite directions. With an automatic, the torque converter handles the difference, and the electronics might even prevent it from actually shifting until it's nearly stopped... But this is just my naive guesswork.
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R..Apr 28 '13 at 2:58

5 Answers
5

A lot of newer cars are smart about shifting (they have electronic solenoids to control the hydraulics). I can put my 2001 Nissan Pathfinder in reverse at 50 MPH, and it's smart enough to not engage, it goes into neutral. However, at speeds below its cutoff point (I've done it at about 15 MPH and regretted it), you can put a lot of stress on the drivetrain if you shift into drive from reverse (or vice-versa). Even more so if you shift and apply the accelerator.

But when you are moving slowly, like 5 MPH or less? No, no harm--at least in the automatics I have experience with. The transmission will shift through neutral first, relieving any preload. Then when it shifts to drive or reverse, the torque converter will take the mismatched speeds (RPM), that's what it's designed to do. The problems arise when the torque converter engages strongly due to a high RPM difference, and this can shock the drivetrain and break stuff.

In manuals it all depends on how long you slip the clutch. I could make a change from reverse to 1st at 20 MPH be smooth. At least, as long as the clutch and synchronizers last.

I drive a manual, and most of my driving is by feel. I can definitely tell that doing this at higher Reverse speeds is A Bad Thing - but it's nice to know I can likely get by with 5 mph or so, just enough that lazy me doesn't have to stop completely. (also random note, I drive a '95 Ford Escort with 200,000+ mi on it, with the original clutch! Learned that the other day from the previous owner.)
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SunyaatAug 3 '12 at 13:43

I assumed you had an automatic. Oops. With a manual, it puts a lot of wear on the synchronizers to catch up the difference in speed, the larger the worse it is.
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NickAug 3 '12 at 15:06

Thanks for the update - very good to know! :)
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SunyaatAug 3 '12 at 15:21

I have always thought changes of direction while moving was a bad idea. It doesnot matter if it an automatic or standard shift,small car or large truck. Doing this puts tremendous stresses on the drive train. All mechanical parts are built with clearances between moving parts. When you ask them to change direction while moving the parts act as hammers as they are accelerating prior to contact. If you pushed a stationary drinking glass with a hammer it would not break. If the glass was rolling and you wanted to change its direction with a hammer I think the out come would be obvious. It may seem like a less abrupt change with a manual shift because the clutch is slipping and absorbing the energy. So instead of shocking the U joints you are wearing out the clutch.

In the automatics I've been in where people do that, it's a rather dramatic bang/clunk if shifted from Reverse to Drive while still rolling backwards. I hate riding with people that do that, makes me cringe everytime. I hear newer cars are smarter about it, but it still sounds like a bad idea. I hate to risk my transmission on a sensor that might fail when it's so easy to just not do that...

On (typical) manuals, it doesn't cause any large loads, but you have to slip the clutch more to get moving again (as well as the synchros having to do a bit more work). So, it does cause slightly more wear. Beware if you have a race transmission with straight cut gears and no synchros though, that would be bad news going reverse to drive while rolling backwards. Most manuals do not have synchros on reverse, so rolling forward and shifting into reverse is a no-no on they typical manual.

You can do a lot of things in life and get away with it. But why chance it.

If you're rich and you pay others to fix your mistakes, have at it.
But if you're poor and that's your transportation, why in the hell would you risk your transmission?

When my mom paid for everything in my life, I used to rev up my car and throw it in gear to spin the tires. If you would have asked me if it's OK to do that, I would have said, sure, I do it all the time.

Long story short, I blew out the transmission doing that one day.

Now ask me if it's okay to do it! It's real bad on the pump I would say, among other things.